In a typical cellular radio system, wireless terminal(s) communicates via a Radio Access Network (RAN) to one or more Core Networks (CN). The wireless terminal is also known as mobile station and/or User Equipment (UE), such as mobile telephones, cellular telephones, smart phones, tablet computers and laptops with wireless capability. The user equipment's may be, for example, portable, pocket-storable, hand-held, computer-comprised, or car-mounted mobile devices which communicate voice and/or data via the RAN. In the following, the term user equipment is used when referring to the wireless terminal.
The RAN covers a geographical area via cells, where each cell is being served by a base station, e.g., a Radio Base Station (RBS), which in some networks is also called NodeB, B node, evolved Node B (eNB) or Base Transceiver Station (BTS). The term base station will be used in the following when referring to any of the above examples. A cell is a logical entity. The cell has been assigned a set of logical resources, such as radio channels that provides for radio communication in a geographical area. The base station at a base station site physically realizes the logical cell resources such as transmitting the channels. From a user equipment perspective the network is represented by a number of cells.
In some versions, particularly earlier versions, of the RAN, several base stations are typically connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC). The RNC, also sometimes termed a Base Station Controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more CNs.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. Universal Terrestrial Radio Access Network (UTRAN) is essentially a RAN using WCDMA for user equipment's. The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based RAN technologies.
Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base stations are connected directly to a CN rather than to RNCs. In general, in LTE the functions of a RNC are performed by the base station. As such, the RAN of an LTE system has an essentially “flat” architecture comprising base stations without reporting to RNCs.
LTE was introduced in 3GPP with its release 8. Release 9 and release 10 are later releases of LTE. For example, release 8 may be referred to as e.g. rel-8, release 8, LTE release 8 or 3GPP release 8. The terms “codeword,” “layer,” “precoding,” and “beam forming” have been adapted specifically for LTE to refer to signals and their processing. A codeword represents user data before it is formatted for transmission. The term “layer” is synonymous with stream. For Multiple Input Multiple Output (MIMO), at least two layers must be used. Up to eight layers are allowed. The number of layers is always less than or equal to the number of antennas on the base station. Precoding modifies the layer signals before transmission. This may be done for diversity, beam steering, or spatial multiplexing. Beam forming modifies the transmit signals to give the best Carrier to Interference-plus-Noise Ratio (CINR) at the output of the channel.
LTE comprises multi-antenna transmission. The different multi-antenna transmission schemes in LTE correspond to different transmission modes (tm). The different transmission modes differ with regards to the specific structure of antenna mapping, Reference Signals (RS) used for demodulation, and channel state information (CSI) feedback. LTE supports nine transmission modes for the DownLink (DL) transmission path. The nine transmission modes are as follows:                Mode 1: Single antenna port.        Mode 2: Transmit diversity.        Mode 3: Open-loop spatial multiplexing.        Mode 4: Closed-loop spatial multiplexing.        Mode 5: Multi User-MIMO (MU-MIMO).        Mode 6: Closed-loop spatial multiplexing, single layer.        Mode 7: Single antenna port, user equipment specific RS.        Mode 8: Single or dual-layer transmission with user equipment specific RS.        Mode 9: Closed loop Single User-MIMO (SU-MIMO) and MU-MIMO.        
Modes 1-8 are for LTE release 8/9, and mode 9 is for LTE-Advanced, also referred to as LTE release 10. Mode 9 is a multilayer transmission mode to support closed-loop SU-MIMO and enhanced MU-MIMO support. The transmission modes supported by the base station is decided by the vendor of the base station upon implementation of the base station. The base station configures the transmission mode in the user equipment and hence knows which transmission mode the user equipment is configured with. The spatial multiplexing in mode 3 and 4 is a transmission technique in MIMO wireless communication to transmit independent and separately encoded data signals from each of the multiple transmit antennas.
In LTE, Hybrid Automatic Repeat reQuest (HARQ) with incremental redundancy is used. HARQ is a technique that enables faster recovery from errors in communication networks by storing corrupted data packets in the receiving device rather than discarding them. Even if retransmitted data packets have errors, a good data packet may be derived from the combination of bad ones. Instead of re-transmitting the same portion of the codeword, different redundancy versions are re-transmitted, yielding an extra gain over Chase combining. Ideally, a full soft buffer is available at the receiver side such that the received soft values/bits for the entire codeword may be stored. A soft bit is a representation of a measure of how likely it is that the bit is a 0 or 1. However, due to the user equipment complexity and cost concerns, the soft buffer size in a user terminal is limited. For higher rate transmissions, where larger codewords are sent from the transmitter, the user equipment may have only limited buffer space and is not able to store the complete codeword. Therefore, the base station and the user equipment should agree on the soft buffer size. Otherwise, the base station may transmit coded bits the user equipment is not able to store, or worse, the user equipment does not know these are other bits and confuses them with bits it has stored previously.
FIG. 1 depicts in simplified form a complete codeword and also how many soft bits the user equipment may store. FIG. 1 illustrates an enclosed transport block and coded bits stored by the user equipment, i.e. soft buffer size. As seen in FIG. 1, the complete codeword comprises systematic bits and parity bits. The soft buffer size comprises all systematic bits and some of the parity bits of the complete codeword. Parity bits are bits generated by channel coding using a Forward Error Correction (FEC) channel code such as a turbo code. Parity bits may be used in the receiver to detect and correct transmission errors within the error correction/detection capabilities of the channel code. If the base station and the user equipment have the same understanding about the soft buffer size, the base station will not transmit coded bits which the user equipment is not able to store. Instead, the base station only takes those coded bits stored by terminal and uses those bits for (re)transmissions. This is depicted by the circular buffer shown in FIG. 2. The term circular buffer refers to an area in a memory which is used to store incoming data. When the buffer is filled, new data is written starting at the beginning of the buffer and overwriting the old. The code word, i.e. the systematic bits and the parity bits, are stored in the circular buffer. FIG. 2 illustrates the bits used in a first transmission and re-transmissions, derived from the circular buffer. The size of the circular buffer matches the soft buffer size of the user equipment. The complete circle corresponds to the soft buffer size and not to the entire codeword. In the first transmission, depending on the code rate, some or all systematic bits, and none or some parity bits are transmitted. In a retransmission the starting position is changed and bits corresponding to another part of the circumference, e.g. another point in the circular buffer, are transmitted.
In LTE release 8 using Frequency-Division Duplexing (FDD), each user equipment has up to 8 HARQ processes. Each HARQ process may comprise up to two sub-processes for supporting dual-codeword MIMO transmissions. LTE release 8 divides the available soft buffer equally into the configured number of HARQ processes. Each of the divided soft buffers may be used to store soft values of the received codewords. In case of dual-codeword MIMO transmission, the divided soft buffer is further divided equally to store the soft values of the two received codewords.
A Soft Buffer (SB) allocation for the single-codeword transmission modes is illustrated in FIG. 3. FIG. 3 illustrates eight allocated soft buffers, where SB0 illustrates a first soft buffer for a first codeword, SB1 illustrates a second soft buffer for a second codeword, SB2 illustrates a third soft buffer for a third codeword etc. FIG. 3 shows soft buffer allocation in LTE release 8 when the Physical Downlink Shared Channel (PDSCH) transmission mode is other than mode 3, 4 or 8. It may be observed that there is a buffer reserved for each codeword.
The soft buffer allocation for the dual-codeword transmission modes is illustrated in FIG. 4. FIG. 4 illustrates sixteen allocated soft buffers, where SB0a illustrates a first buffer for a first codeword, SB0b illustrates a second buffer for a second codeword, SB1a illustrates a third buffer for a third codeword, SB1b illustrates a fourth soft buffer for a fourth codeword etc. The soft buffer applies to a codeword. The codeword is a term used for the coded bits associated with a transport block. FIG. 4 shows soft buffer allocation in LTE release 8 when the PDSCH transmission mode is mode 3, 4 or 8.
The buffer reserved for each codeword is only half of the previous operating case. The soft buffer limitation problem is particularly acute in dual-codeword MIMO transmission operations. This limitation reduces the effectiveness of soft combining gains from incremental redundancy retransmissions.
Carrier Aggregation. The LTE release 8 supports bandwidths up to 20 Mega Hertz (MHz). However, in order to meet the International Mobile Telecommunications-Advanced (IMT-Advanced) requirements, 3GPP has initiated work on LTE release 10. One part of LTE release 10 is to support bandwidths larger than 20 MHz. An important requirement for LTE release 10 is to assure backward compatibility with LTE release 8, including spectrum compatibility. As a result, a carrier or LTE release 10, which is wider than 20 MHz, should appear as a number of smaller LTE carriers to a user equipment of LTE release 8. Each such carrier may be referred to as a component carrier. For early LTE release 10 deployments, it may be expected that there will be a smaller number of LTE release 10 capable user equipment's compared to many LTE legacy user equipment's. Therefore, it is desirable to assure an efficient use of a wide carrier by legacy user equipment's, which means that it should be possible to implement carriers where legacy user equipment's may be scheduled in all parts of the wideband LTE release 10 carrier. One way to achieve this would be using carrier aggregation. The term legacy refers to that the user equipment continues to be used, typically because it still functions for the users' needs, even though newer technology or more efficient technology is available.
Carrier aggregation implies that a LTE release 10 user equipment may receive multiple component carriers, where the component carriers have, or at least can have, the same width as a LTE release 8 carrier. An example of carrier aggregation is illustrated in FIG. 5. The x-axis of FIG. 5 denotes the width of the spectrum 501 used for the five component carriers 505 and the y-axis defines the energy per frequency unit 510.
Soft Buffer Operation in Carrier Aggregation. In LTE each component carrier operates with its own set of HARQ processes. Since the total soft buffer memory needs to be shared among component carriers, the soft buffer size per component carrier may vary depending on the number of configured component carriers and the configured MIMO transmission modes for each component carriers. The available soft buffer size for each codeword also depends on how the soft buffer is divided and allocated amongst all codewords.
Multi-Antenna Support in LTE. Multi-antenna capabilities are included already in LTE release 8, and are important enablers for high data rates, improved coverage and capacity. The multiple antennas at transmitters and receivers may be used in different ways. Diversity techniques are used to improve the robustness of the link, and beamforming techniques may be used to improve the coverage. Spatial multiplexing provides means to enhance the spectral efficiency of the link and may improve the performance of the whole system if properly designed. Peak rates may be substantially increased using spatial multiplexing and may ideally be increased proportionally to the minimum number of transmit and receive antennas of the link provided that the Signal-to-Noise-Ratio (SNR) is high enough and that the channel conditions are beneficial. Realistic gains are highly channel dependent, e.g. they require a high SNR and beneficial interference situations of the relevant link, but may be substantially improved provided that the SNR is sufficiently high. Examples are low system load scenarios or when the user equipment is close to the cell center.
The downlink of LTE release 8 supports SU-MIMO spatial multiplexing of up to four layers via codebook based precoding. In addition, transmit diversity modes as well as beamforming with single-layer transmission are supported in the downlink of LTE release 8. In LTE release 9, an enhanced downlink transmission mode is introduced in which the beamforming functionality is extended to also support dual-layer transmission, and in which MU-MIMO operation is offered where different layers are transmitted to different user equipment's. The LTE release 8/9 uplink multi-antenna support is limited to user equipment antenna selection, which is optional in all user equipment categories. The user equipment categories will be described in more detail below.
User Equipment Category Signalling. User equipment's may be categorized in different user equipment categories, called UE categories or UE classes, which defines the overall performance and capabilities of the user equipment. The user equipment categories are needed to ensure that the base station may communicate correctly with the user equipment. By letting the base station know the user equipment category, it is able to determine the performance of the user equipment and communicate with it accordingly. Accordingly, the base station will not communicate beyond the performance of the user equipment. Different values of a buffer size are associated with each user equipment category. The user equipment category may be referred to as UE category in some of the figures.
In LTE release 8/9, there are five user equipment categories, 1-5. LTE release 10 has three additional categories, 6-8, i.e. in all eight user equipment categories for the releases, as shown in Table 1 below. The column 1 comprises the user equipment categories 1-8. The column 2 illustrates that user equipment categories 1-5 are related to LTE release 8/9 and the column 3 illustrates that user equipment categories 1-8 are related to LTE release 10.
TABLE 1User Equipment Categories1. User2. LTE3. LTEEquipmentreleasereleaseCategory8/9101XX2XX3XX4XX5XX6X7X8X
Table 2 below shows user equipment categories related to LTE release 8/9/10. Column 1, i.e. the left most column, comprises the LTE releases, and column 2 comprises the user equipment categories 1-8. Column 3 comprises the maximum number of Downlink-Shared Channel (DL-SCH) transport block bits received within a Transmission Time Interval (TTI). Column 4 comprises the maximum number of bits of a DL-SCH transport block received within a TTI. Column 5 comprises the total number of soft channel bits. Column 6 comprises the maximum number of supported layers for spatial multiplexing in DL. As mentioned earlier, spatial multiplexing is a transmission technique in MIMO wireless communication to transmit independent and separately encoded data signals from each of the multiple transmit antennas.
TABLE 2User Equipment categories supported in LTE release 8/9/103. Maximum6. Maximumnumber of4. Maximum numbernumber ofDL-SCHof bits 5. Totalsupportedtransportof a DL-SCHnumberlayers for2. Userblock bitstransport blockof softspatial1. LTEEquipmentreceivedreceived withinchannelmultiplexingreleaseCategorywithin a TTIa TTIbitsin DLRelease1102961029625036818/9/102510245102423724823102048753761237248241507527537618270722529955214977636672004Release 106301504149776 (4 layers)36541442 or 4only 75376 (2 layers)7301504149776 (4 layers)36541442 or 4 75376 (2 layers)829985602998560359827208
As seen from table 2, the different user equipment categories are associated with e.g. a total number of soft channel bits and a maximum number of supported layers for spatial multiplexing in downlink. A user equipment of LTE release 10 supports transmission mode 9 and an 8 layer transmission is only possible with transmission mode 9.
The definition of user equipment categories related to LTE release 10 builds upon the principles used in LTE release 8/9, where the number of user equipment categories is limited to avoid fragmentation of user equipment implementation variants in the market. The user equipment categories related to LTE release 10 are defined in terms of peak rate, ranging from 10, 50, 100, 150 and 300 Mbps up to about 3 Gbps in the downlink.
Different realizations of the peak rates are possible within a user equipment category. For example, in user equipment categories 6 and 7, it is possible to either support two layers of MIMO together with carrier aggregation of 40 MHz, or four layers of MIMO with a single carrier of 20 MHz. Both configurations support up to 300 Mbps. The user equipment categories related to LTE release 8/9 are reused, supporting, e.g. aggregation of two component carriers with up to 10 MHz bandwidth each for a user equipment of user equipment category 3. The user equipment signals the number of supported MIMO layers for each band combination, in line with the requirements from the user equipment categories. While it is expected that additional user equipment categories may be defined in the future, LTE release 10 supports a high-end user equipment category combining aggregation of five component carriers of 20 MHz each with eight layer MIMO, which supports a total peak data rate of about 3 Gbps for LTE-Advanced.
A LTE release 10 user equipment may indicate two user equipment categories: a user equipment category relating to LTE release 8/9, e.g. user equipment category 5, and a user equipment category relating to LTE release 10, e.g. user equipment category 8. A problem is that a base station supporting LTE release 8/9 may only detect the user equipment category relating to LTE release 8/9, while a base station supporting LTE release 10 would detect both the user equipment category relating to LTE release 8/9 and the user equipment category relating to LTE release 10. A similar problem may also occur if the user equipment relates to a user equipment category 6/7 in a base station supporting LTE release 10, when it indicates a user equipment category 4 relating to LTE release 8/9.
As the user equipment is not aware of the LTE release of the base station it does not know whether to operate according to the user equipment category relating to LTE release 8/9, e.g. category 5, or the category relating to LTE release 10, e.g. category 8. This has serious consequences, as the user equipment would operate differently, depending on the user equipment category. As may be seen from the Table 2, the soft buffer sizes for the two user equipment categories differ. This may result in that the user equipment and base station using different soft buffer size, which leads to corruption of the channel coding, as the rate matching in the base station and the user equipment would be done according to different soft buffer sizes. As a result, the user equipment will not be able to receive data on the PDSCH. The coding protection will not help as the encoded bits will be interleaved into the wrong place.